Recently, with advances in digital technology, electronic devices such as portable information devices and home information appliances have been developed to provide higher functionality. As the electronic devices have been developed to provide higher functionality, development of further miniaturized and higher-speed semiconductor elements is advancing at a high pace. Among them, the use of large-capacity nonvolatile memories which are typified by a flash memory has been expanding at a rapid pace. Furthermore, as next-generation new nonvolatile memories which have a potential to replace flash memory, research and development of what is known as a variable resistance nonvolatile memory device which uses a variable resistance element is advancing. As defined herein, the variable resistance element refers to an element which has a characteristic in which a resistance value changes reversibly in response to electric signals and is further able to store information corresponding to the resistance value in a nonvolatile manner.
As an example of a large-capacity nonvolatile memory device incorporating the above-described variable resistance elements, a crosspoint nonvolatile memory device has been proposed. For example, disclosed in Patent Literature (PTL) 1, as a memory unit, is a nonvolatile memory device having a configuration in which a variable resistance film and a diode element as a switching element are used.
Shown in FIG. 11A and FIG. 11B is a configuration of a nonvolatile memory device containing a conventional variable resistance element disclosed in PTL 1. FIG. 11A is a perspective view showing the configuration of the conventional nonvolatile memory device. Shown is the configuration of a crosspoint memory cell array 200 made of memory cells 280, each of which are formed at a crosspoint of a bit line 210 and a word line 220. FIG. 11B is a cross-sectional view showing the configuration of the memory cell provided in the conventional nonvolatile memory device. Shown is the configuration, taken along the direction of a bit line 210, of the memory cell 280, the bit line 210, and the word line 220.
As shown in FIG. 11A and FIG. 11B, a variable resistance element 260 includes a variable resistance layer 230 for storing information according to a change in electric resistance in response to electric stress applied thereto, and an upper electrode 240 and a lower electrode 250 placed on either side of the variable resistance layer 230. Formed on the variable resistance element 260 is a two-terminal nonlinear element 270 having a nonlinear current-voltage characteristic for flowing a current bidirectionally. The memory cell 280 is formed of a series circuit including the variable resistance element 260 and the nonlinear element 270. The nonlinear element 270 is a two-terminal element having a nonlinear current-voltage characteristic similar to that of a diode in which a current changes inconstantly with respect to a voltage change. Moreover, the bit line 210 serving as an upper line is connected electrically to the nonlinear element 270, and the word line 220 serving as a lower line is electrically connected to the lower electrode 250 of the variable resistance element 260. Because current flows bidirectionally when rewriting the memory cell 280, a varistor (ZnO or SrTiO3) having a current-voltage characteristic which is bidirectionally symmetric and nonlinear is used as the nonlinear element 270. With the above configuration, it is possible to flow a current with a current density of 30 kA/cm2 or higher which is required for rewriting for the variable resistance element 260.